4C50.20 - Critical Opalescence

See paragraph one in the procedure section.
Code Number:
Demo Title:
Critical Opalescence
Change of Refractive Index with Phase Change
Area of Study:
Hot water bath, Hot plate, Erlenmeyer flask with stopper, Hexane, Methanol, Thermometer.

Critical opalescence is caused by a large change in the refractive index. In this experiment we mix hexane and methanol in the molar ratio of .435 moles to .665 moles respectively. Methanol has 40.45 ml / mole. Hexane has 130.55 ml / mole. This means that 17.60 ml of methanol and 85.51 ml of hexane will give you the .435 : .665 mole ratio. The critical opalescence will occur at 42.4 C. The easiest way to do this is to heat the mixture above 42.4 C. so that the mixture is clear and appears as one liquid. Let the mixture cool and observe the critical opalescence at the transition temperature. Depending on existing pressure and experimental conditions this may vary slightly from the 42.4 C reported in the literature. As the mixture continues to cool the two liquids will separate into distinct layers.

  • Clarendon Press, "The Theory of Critical Phenomena", p. 1-13.

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